(((hugs))) Kara, I would be nervous given M's history as well. I know F started out with a bit of a sensitive tummy, but I think because we took it slow and let him decide when to eat his system matured well and now he tolerates pretty much anything.
Do you know for sure if C is intolerant to anything? What does his doctor say? I would think you could start trialling some things, one at a time of course and giving lots of time in between.
And I don't think F's eating habits are amazing. You should hear what some of the other LOs on our BCs eat, I think Kim's Jack and Lyndsey's Colin with both eat just about anything offered to them in any way, shape or form! I offer a lot, but F doesn't eat a heck of a lot. But I do let him decide if he wants to eat it or not.